On 18.2.2014 02:23, KONDO Mitsumasa wrote: > Hi, > > I don't have PERC H710 raid controller, but I think he would like to > know raid striping/chunk size or read/write cache ratio in > writeback-cache setting is the best. I'd like to know it, too:) We do have dozens of H710 controllers, but not with SSDs. I've been unable to find reliable answers how it handles TRIM, and how that works with wearout reporting (using SMART). The stripe size is actually a very good question. On spinning drives it usually does not matter too much - unless you have a very specialized workload, the 'medium size' is the right choice (AFAIK we're using 64kB on H710, which is the default). With SSDs this might actually matter much more, as the SSDs work with "erase blocks" (mostly 512kB), and I suspect using small stripe might result in repeated writes to the same block - overwriting one block repeatedly and thus increased wearout. But maybe the controller will handle that just fine, e.g. by coalescing the writes and sending them to the drive as a single write. Or maybe the drive can do that in local write cache (all SSDs have that). The other thing is filesystem alignment - a few years ago this was a major issue causing poor write performance. Nowadays this works fine, most tools are able to create partitions properly aligned to the 512kB automatically. But if the controller discards this information, it might be worth messing with the stripe size a bit to get it right. But those are mostly speculations / curious questions I've been asking myself recently, as we've been considering SSDs with H710/H710p too. As for the controller cache - my opinion is that using this for caching writes is just plain wrong. If you need to cache reads, buy more RAM - it's much cheaper, so you can buy more of it. Cache on controller (with a BBU) is designed especially for caching writes safely. (And maybe it could help with some of the erase-block issues too?) regards Tomas -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
